itimpi

With those settings nothing is going to be recorded as those settings just set up the server to be listening.  There are 2 ways to get something recorded:
use the Mirror to Flash option (normally the easiest).   In this case the results are put into the ‘logs’ folder on the flash drive.. put the IP address of the Unraid server into the Remote syslog server setting.   In this case the results are put into folder specified in the settings.

I would expect it is most likely to be a mapping in a container that needs fixing.

This is not an option in the current Unraid releases as you cannot move files between pools - only to/from a pool and the main Unraid array.
 
it is likely that this restriction will disappear in the Unraid 6.13 release where we have been told that the current Unraid array becomes just another pool type, but there is no ETA when this will be available, and there is always the chance that things do not turn out as expected.

You probably want to use the process documented here in the online documentation accessible via the ‘Manual’ link at the bottom of the GUI or the DOCS link at the top of each forum page.

Not as you describe.   A file is meant to only be in one place, either array or a pool.   The closest you will get is using a plugin like Appdata Backup to periodically backup the appdata share to another location on the array.
 

Unraid only supports a file being in 1 location (I.e. on a pool OR on the array) at a given point in time.

Since Preclear is not a standard part of Unraid the rebuild process does not check whether the disk has a Preclear signature.    I agree that it could as an optimisation but that would be a new feature/enhancement.

I do not think that Unraid will let you add a new parity and a new array disk in one step so you would need to do them one after the other (in any order).

What you did would work but is a bit error prone.
 
A better process is something like
Use Tools->New Config and select the option to keep all assignments return to the Main tab and change the drive assigned to cache if you parity, check the Parity is Valid checkbox to avoid rebuilding parity.   You can then ignore the warning about overwriting parity as it does not take into account you checked this option. start the array and then format (if needed) the new cache drive.

Looks like it has a old firmware version, so you might want to consider updating it to the most recent version. 

This is all configurable.
 
You can individually configure for each share which pool (if any) is used for caching purposes. 
You can also configure where docker containers and VMs should be placed.
 
You can change things around later if you change your mind about where things should go, although depending on the change made you may then have to manually move some data to match the new setup.

That is normal.   It indicates how much of the ZFS Arc us being used.

You cannot do that using ZFS.  You would have to replace both drives at the same time which means any existing data first has to be copied elsewhere.   This is one reason for using BTRFS in pools - it is much more flexible about adding drives (particularly of mixed sizes).

If it replaces a drive then it will be rebuilt with that drives contents.   Its current contents are irrelevant as the rebuild process overwrites every sector in the drive.  
 
If it is added as a new drive then any existing contents will first be cleared so again any existing content is irrelevant.

Click on the pool on the Main tab and change it to use the Single profile.

How are you placing the files into the Media share?  If it is via a docker container then you may be encountering the behaviour described here in the online documentation accessible via the ‘Manual’ link at the bottom of the GUI or the DOCS link at the top of each forum page.
 
you are correct in the the terminology changed in the 12.0 release (although the functionality has not).  The new terminology is described in some detail in those release notes.

Do you have Turbo Write enabled?    If so this would explain your symptoms as a write to any drive requires all drives to be spinning.

You have to decide which one is the most current (probably the one on the cache, and then you can delete the other one.
 
Note that the high utilization of docker.img is unrelated to this issue - it tends to mean that the way you have plex configured it is writing to a location internal to the docker.img file rather than to an external mapped location.

The network interface seems to be continually going up and down.   Have you tried rebooting the router.
 
i notice that you seem to have Jumbo frames enabled.   In my experience these tend to cause more problems than they are worth and provide negligible performance gain.

There is a known problem with docker that can cause this that had been fixed in the 6.12.3 release.

How is the disk in question connected?   In the diagnostics I see:
Jul 26 19:03:07 unraid1 kernel: sd 2:0:3:0: attempting task abort!scmd(0x000000002f39e8d5), outstanding for 7339 ms & timeout 7000 ms Jul 26 19:03:07 unraid1 kernel: sd 2:0:3:0: [sdh] tag#1801 CDB: opcode=0x85 85 06 20 00 d8 00 00 00 00 00 4f 00 c2 00 b0 00 Jul 26 19:03:07 unraid1 kernel: scsi target2:0:3: handle(0x000c), sas_address(0x4433221105000000), phy(5) Jul 26 19:03:07 unraid1 kernel: scsi target2:0:3: enclosure logical id(0x500605b004e9ec1f), slot(6) Jul 26 19:03:07 unraid1 kernel: sd 2:0:3:0: task abort: SUCCESS scmd(0x000000002f39e8d5) which suggests some sort of connection issue.

The Unraid GUI needs to be on eth0.   Maybe this is your problem and you need to set the 2.5G connection to be eth0?  Also, have you made sure the 1G and 2.5G connections are on different subnets?

Booting up on another PC will not do any damage on that machine as you do not try to configure anything.
 
it might be worth downloading the zip file for the Unraid release you are using and extract all the bs* type files overwriting the ones in the root of the flash drive just in case there is a problem reading one of them.

Sounds like there may be some issue successfully reading the system files off the flash drive.   
 
I would suggest you try downloading the zip file for the release you are using from the Limetech download site and extract all the bz* type files overwriting the ones in the top level of the flash drive as this sometimes seems to help.

I would suggest that you update to the current stable release (6.12.3 with 6.12.4 coming soon) to see if the problem still exists.